Are you talking of the open A Major chord ? I think thats what I understood from what you said. What you can do instead of placing your 3 fingers on the B the G and the D string, catch them all with just your index. Just like if you wanted to bar the 2nd fret, but do it for only those 3 strings. The tricky part is not to mute the high E string, so you have to drop you thumb lower behind the neck. That way you will be able to bend your index more so you dont touch the high E. You can also try it with 2 fingers. The index plays the D string and the middle finger plays the G and B.